Add contribution.md

Change-type: patch
Signed-off-by: Chris Crocker-White <[email protected]>

Author: chrisys

CONTRIBUTING.md

@@ -0,0 +1,13 @@
+# Contributing
+
+## Semantic versioning and commit messages
+
+The balenaDash version numbering adheres to [Semantic Versioning](http://semver.org/). The following
+header/row is required in the body of a commit message, and will cause the CI build to fail if absent:
+
+```
+Change-type: patch|minor|major
+```
+
+Version numbers and commit messages are automatically added to the `CHANGELOG.md` file by the CI
+build flow, after a pull request is merged. It should not be manually edited.